In wireless communication systems, a received radio frequency (RF) signal may be converted to an intermediate frequency (IF), and then from IF to a baseband signal, where the IF may be in the megahertz range. For certain systems, it may also be possible to convert directly from RF to baseband. Generally, the RF signal may be mixed with a local oscillator signal that results in two (double) sideband signals that are the sum of the frequencies of the two signals and the difference of the frequencies of the two signals, where the difference is often called ‘beat frequency’. The lower frequency component is typically the signal that is required for further processing of the signal. One of the two sideband signals may be chosen as an IF signal, and this IF signal may be the same for all received RF signals. Therefore, a radio that may receive a plurality of channels, such as a Wireless LAN radio, may tune to a particular frequency corresponding to one of 11 standard channels by changing the local oscillator signal frequency such that the IF remains constant. With a constant IF, most of the receive path may be common in the receiver.
Today, much of radio receiver development may be driven mostly by a great demand for mobile wireless communication devices, including handsets. With the ever-decreasing size of mobile handsets, capacities of smaller batteries may be an issue. As most of these handsets may utilize complementary metal-oxide semiconductor (CMOS) technology for analog-to-digital conversion, and for much of the processing of voice and data signals, a very important factor to consider is that it may be advantageous for CMOS devices to operate at lower frequencies. This may be crucial since CMOS devices have power dissipation directly related to the speed at which the CMOS devices switch. The faster the frequencies, the faster the CMOS device switching speed, and therefore, the greater the amount of power consumed. Therefore, receivers may be designed to downconvert the high frequency RF, which may be in gigahertz range, to a lower frequency, preferably to a baseband frequency, as quickly as possible.
Besides the operation of frequency downconversion, the demodulation circuitry also separates the in-phase (I) channel from the (Q) quadrature channel. The received RF signal may be written as the sum of a component modulated onto a cosine at the carrier frequency and a component modulated onto a sine at the carrier frequency. The component modulating the cosine is termed the in-phase component and the term modulating the sine is termed the quadrature component since the sine wave is equivalent to a cosine wave with a 90 degrees phase shift.
The separation of the channel may be achieved by multiplying the received signal with the local oscillator as described above. The baseband component of this operation may then be processed as the I channel. To obtain the Q channel, the received signal can be multiplied with the local oscillator signal that is phase shifted by 90 degrees.
Another important factor to consider may be the signal integrity in the signal path. Because signals received at a receiver's antenna may be very weak, for example, six millivolts (6 mV), the first component to process the received signal may be a low noise amplifier (LNA) that is designed to amplify signals while adding very little additional noise to the signal being amplified. The amplified signal may be filtered to attenuate undesired signals, amplified further to increase the strength of the signal, and mixed with local oscillator signals to downconvert to lower frequencies. Factors such as process, voltage and temperature (PVT) variations may also result in a DC offset.
Due to limitations on the power consumption, in particular for the mobile communications terminal, it is crucial to minimize the number of components and the die area required for analog RF circuitry. Fewer components, in particular, active components, may also help to keep heat dissipation down and reduce power consumption when the circuits are idle, due to less biasing currents. Also very significant is that certain analog components may take up disproportionate amounts of space on integrated circuits and their use is therefore to be kept to a minimum. Examples are inductors and large capacitors.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.